A typical known receiving antenna includes a parabolic reflector and a corresponding feed horn to guide energy received from a transmitting antenna into a circular waveguide. The energy propagates through the waveguide to an orthomode transducer, which simultaneously extracts horizontally and vertically polarized energy. Such antennas are used in many microwave communications applications, including ground relays and geosynchronous communications satellites, which simultaneously transmit both vertically polarized linear signals and horizontally polarized linear signals on the same frequency allocation. In such applications, it is advantageous to use a receiving antenna that can simultaneously receive both of the respective polarizations, thereby reducing cost complexity and minimizing the space required at the facility at which the receiving antenna is installed.
Referring to FIG. 1, a known Newtonian feed antenna system 20 is configured to receive respective horizontally and vertically polarized signals 22 and 24 from a geosynchronous communications satellite transmitter (not shown) along an axis 26 of the antenna 20. The antenna system 20 generally includes a true parabolic reflector 28 and a feed assembly 30. The reflector 28 includes a parabolic arc, which causes the respective signals 22 and 24 to reflect from the surface of the reflector 28 towards a focal point 32, as best depicted in FIG. 2. The feed assembly 30 includes a circular feed horn 34, circular waveguide 36 and orthomode transducer (not shown). The feed assembly 30 is supported by a feed assembly support 38, such that the feed horn 34 is supported at the focal point 32. Thus, the respective signals 22 and 24 that are directed towards the focal point 32 from the reflector 28 are conveyed down the feed horn 34 to the waveguide 36, where they are extracted by the orthomode transducer for processing by further receiving circuitry (not shown). In this manner, a single feed antenna is provided with dual-polarization capability.
The dual polarization capability of the antenna 20, however, presents a problem in that the E-field of a linearly polarized energy distribution across the aperture of a typical feed horn is different in respective vertical and horizontal planes. FIG. 4 shows a vertically polarized E-field 40 at an aperture 42 defined by a rim 44 of the circular feed horn 34. For ease of illustration, the aperture 42 is depicted as having respective orthogonal X-, Y- and Z- axes, with the X- and Y- axes being coplanar with the aperture 42 and the Z-axis being perpendicular to and passing through the center of the aperture 42. As shown in FIG. 4A, the magnitude of the E-field 40 is fairly uniform along the X-axis (vertical plane) and terminates at full strength at the rim 44. As shown in FIG. 4B, the magnitude of the E-field 40 along the Y-axis (horizontal plane) is maximum at the Z-axis and terminates to zero at the rim 44.
As depicted in FIGS. 5A and 5B, the differing E-field 40 across the aperture 42 produces a horn radiation gain pattern 48 having a beam width (.theta.X) as measured in the vertical plane and a beam width (.theta.Y) as measured in the horizontal plane, which are respectively different. In the vertical plane, where the E-field 40 across the aperture is larger (from rim to rim), the resulting beam width (.theta.X) of the horn radiation gain pattern 48 is narrower. In the horizontal plane, where the E-field 40 across the aperture 42 is smaller (zero at each rim), the resulting beam width (.theta.X) of the horn radiation gain pattern 48 is broader.
Referring to FIGS. 5A and 5B, the horn radiation gain pattern 48 produced by the feed horn 60 is directed towards the surface of the reflector 28 and appears on the reflector 28 in the form of a gain contour 50 (depicted in FIG. 6). The gain contour 50 represents an ideal level of equal gain, typically 1/100th of the peak gain, i.e., -20 dB from the peak gain. The gain contour 50 is optimally coextensive with a rim 52 of the reflector 28, such that the gain measured from the Z-axis to the rim 52 of the reflector 28 decreases gradually enough that the reflector 28 is fully utilized, while still increasing quickly enough that a substantial amount of energy is not radiated outside the reflector rim 52 and lost behind the reflector 28.
As depicted in FIG. 6, however, the gain contour 50 is not coextensive with the reflector rim 12. Rather, the gain contour 50 is elliptical in shape, the gain along the X-axis axis (vertical plane) to decrease too quickly, thereby "underfeeding" the reflector 28 along the X-axis. This mismatch also causes the gain along the Y-axis (horizontal plane) to decrease too gradually, thereby "overfeeding" the reflector 28 along the Y-axis. Because the reflector 28 is "underfed" along the vertical plane, a resulting reflector radiation gain pattern 54 along the vertical plane has a beam width (.phi.X) that is too broad (as depicted in FIG. 7), producing a less than ideal antenna gain. Because the reflector 28 is "overfed" along the horizontal plane, the resulting reflector radiation gain pattern 54 along the horizontal plane has a beam width (.phi.X) that is relatively narrow (as depicted in FIG. 7), but a substantial amount of energy is lost behind the reflector 74, producing a less than ideal antenna gain.
Typically, the feed aperture 42 is sized to adjust the respective breadths of the horn radiation gain pattern 48 as measured in the respective vertical and horizontal planes, i.e., the size of the feed aperture 42 is increased or decreased to respectively narrow or broaden the horn radiation gain pattern 48 in both the vertical and horizontal planes. Because the feed aperture 42 is circular, however, the breadth of the horn radiation gain pattern cannot be adjusted independently for the respective vertical and horizontal planes. Instead, the ideal breadth of the horn radiation pattern in the respective planes and, thus, the ideal gain in the respective planes, must be compromised. Such a problem occurs not only in antenna assemblies such as the antenna system 10, but in any antenna system that employs a circular feed horn to receive a linearly polarized signal.
FIG. 8 depicts a rectangular feed horn 60, which addresses this problem. A vertically polarized E-field 66 is shown at an aperture 62 defined by a rectangular rim 64 of the feed horn 60. For ease of illustration, the aperture 62 is depicted as having respective orthogonal X-, Y- and Z- axes, with the E-field 66 generally polarized parallel and perpendicular to the X- and Y-axes, respectively. The X- and Y-axes are generally coplanar with the aperture 62 and the Z-axis is generally perpendicular to and passes through the center of the aperture 62. As with the circular feed horn 60, the magnitude of the E-field 66 is fairly uniform along the X-axis (vertical plane) and terminates at full strength at the rim 64 (depicted in FIG. 8A), and the magnitude of the E-field along the Y-axis (horizontal plane) is maximum at the Z-axis and terminates to zero at the rim 64 (depicted in FIG. 8B).
Unlike the circular feed horn 60, however, the dimensions of the rectangular feed horn 60 can be adjusted to independently vary the breadth of the horn radiation gain pattern in the respective vertical and horizontal planes. That is, the feed horn 60 has dimensions (a) and (b) in the respective vertical and horizontal planes, which can be independently varied to adjust the horn radiation gain pattern in the respective vertical and horizontal planes. Although the E-field 66 along the horizontal plane terminates to zero at the rim 64, thereby generally creating a broad antenna radiation gain pattern along the horizontal plane, dimension (b) can be made greater than dimension (a) to narrow the antenna radiation gain pattern along the horizontal plane to more closely match the breadth of the antenna radiation gain pattern along the vertical plane. This results in a generally circularized antenna radiation gain pattern that can be more closely matched with a circular reflector.
Adjusting the respective dimensions (a) and (b) of the feed horn 60 to optimize a vertically polarized horn radiation gain pattern will have the opposite effect on a horizontally polarized horn radiation gain pattern, i.e., the horizontally polarized horn radiation gain pattern will become more elliptical. Therefore, adjusting the respective dimensions of a rectangular feed horn will not simultaneously optimize respective vertically and horizontally polarized horn radiation patterns. Thus, a rectangular feed horn is not a solution in a dual polarization application.
This dual polarization problem not only occurs in Newtonian feed antennas, but occurs in other designs as well. Referring to FIG. 9, a known antenna system 80, configured to receive respective first and second polarized signals 82 and 84, includes a ring focus parabolic main reflector 86 and a feed assembly 88. The main reflector 86 includes a parabolic arc that originates from a ring 90 offset from a longitudinal axis 92, which causes the respective signals 82 and 84 to reflect from the surface of the reflector 86 towards a focal ring 94, as best depicted in FIG. 3. The feed assembly 88 includes a circular secondary reflector or "splash plate" 96, a circular feed horn 98, a circular waveguide 100 and an orthomode transducer (not shown). The splash plate 96 is disposed above the focal ring 94, such that the respective signals 82 and 84 reflect off of the splash plate 96, down the feed horn 98 and into the circular waveguide 100, where they are extracted by the orthomode transducer for processing by further receiving circuitry (not shown).
As with the antenna 20, the antenna system 80 presents a problem in that the E-field of a linearly polarized energy distribution across the annular aperture between the feed horn and splash plate in a typical feed assembly is different in respective vertical and horizontal planes. FIG. 10 shows a vertically polarized E-field 102 at an aperture 106 defined by the rim of the circular feed horn 98. For ease of illustration, the annular aperture 104 is depicted as having an axis of revolution around which the angles 0.degree., 90.degree., 180.degree. and 270.degree. are labeled. The E-field 102 is generally polarized along the respective 0.degree. and 180.degree. locations. As shown in FIG. 10A, the E-field 102 at the 90.degree. and 270.degree. locations peaks along the boundary of the annular aperture 104 and terminates to zero at the feed horn rim 106 and splash plate rim 108. As shown in FIG. 10B, the magnitude of the E-field 102 at the 0.degree. and 180.degree. locations is fairly uniform along the boundary of the annular aperture 104 and terminates at full strength at the feed horn rim 106 and splash plate rim 108.
Like the feed assembly 30 of the antenna 20, the feed assembly 88 produces a horn radiation gain pattern with different beam widths in orthogonal planes, resulting in an elliptical gain contour on the main reflector 86 and an inefficient reflector radiation gain pattern.
This problem becomes more significant when designing antennas in which the reflector energy distribution is critical, such as, e.g., multiple reflector noise cancellation antennas, the features of which are described in Lusignan, U.S. Pat. No. 5,745,084, and copending application Ser. No. 08/259,980, filed Jun. 17, 1994, both of which are fully incorporated herein by reference.
Another problem that occurs in the previously described antennas is the occurrence of unintended modes generated at sudden transitions in structures, such as, e.g., a splash plate, feed horn or waveguide. These transitions create unwanted modes that may couple energy from one polarization to another (cross-coupling) or impedance mismatch that may channel energy back out the feed (reflections) instead of guiding energy out through the orthomode transducer. If the length of the waveguide and the distance between the splash plate and the feed horn are relatively great, the deleterious results of the unintended modes will be small. For mechanical reasons, however, the antenna may be less expensive and more acceptable in its application if the feed horn is short. A shorter feed horn, however, can allow unintended modes to couple between sections of the feed and lead to loss and cross-coupling.